title: Causal diagrams, the placebo effect, and the expectation effect.
abstract: Using causal diagrams, a formal research methodology, we analyzed several definitions of placebo and the placebo effect. We conclude that placebo is an ambiguous, redundant term and that the so-called placebo effect conceals far more interesting effects that are attributed to the patient's expectation. Biomedical research will benefit from abandoning the term placebo effect and focusing instead on a deeper understanding of the expectation variable, including its causes, effects, and effect modifiers. This avenue of research should be pursued by observational cohorts that are nested within clinical trials.
authors.affiliation: Division of Epidemiology and Biostatistics, Mel and Enid Zuckerman College of Public Health, University of Arizona, Tucson, AZ, USA.
authors.forename: Eyal
authors.initials: E
authors.lastname: Shahar
authors.forename: Doron J
authors.initials: DJ
authors.lastname: Shahar
keywords: causal diagrams; effect modification; expectation; placebo